The present invention relates to methods for making optical fiber preform starter tubes ("starter tubes"), and optical fiber preforms. More particularly, it concerns a process for making such tubes and preforms by depositing silica with low hydroxyl content through a plasma process.
The prior art teaches various approaches for fabricating silica glass starter tubes, and for making optical fiber preforms. Starter tubes can be formed by heating silica and extruding it through an aperture. Both starter tubes and optical fiber preforms can be made by depositing doped or undoped silica onto a target using one of several techniques such as modified chemical vapor deposition (MCVD), vapor axial deposition (VAD), outside vapor deposition (OVD). Each of these methods starts with providing a rotating target, typically shaped in the form of a tube or a solid rod, and formed from glass, ceramic or one of several other materials. In certain cases, the rod or tube becomes an integral part of the preform but, in other cases, the rod will be removed. A heat source, such as a gas burner or a plasma source is positioned beneath the rotating target. The heat source will provide the required energy for the glass-forming reactions to form glass particles. Depending upon the nature of the process, these deposited glass particles are ready for the next processing, drying and sintering steps such as VAD or OVD processes. If it is an MCVD process, these particles will be fused into vitreous quartz by the same heat source.
When the target is mounted horizontally, the heat source travels along the length of the target to ensure uniform deposition. If the target is a tube, the glass forming particles and materials may be deposited either on the inside surface of the tube, in which case the outer diameter remains constant, or on the outside of the tube, in which case the outer diameter grows.
When the target is mounted vertically, it rotates around its vertical axis, and grows in both radial and axial directions. This results in a substantially cylindrical product whose diameter and length increase as deposition continues.
U.S. Pat. No. 4,224,046 to Izawa et al. teaches a method for manufacturing an optical fiber preform. Two gaseous raw glass materials, oxygen, hydrogen and argon are jetted upwards in a burner towards a vertically mounted, rotating cylindrical start member. Soot-like glass particles are formed by flame hydrolysis and deposited on the lower end of the start member. The start member is gradually withdrawn upwards to maintain a constant spacing between the its growing end and the burner. Upon completion of the deposition, the resulting soot-like glass preform is then dried and sintered to form a transparent glass preform.
U.S. Pat. No. 4,412,853 to Partus discloses an MCVD process to form an optical fiber preform starter tube. The process starts with a horizontally mounted, rotating tubular target formed from glass and having a preselected composition and optical characteristics. A vapor stream is fed through the tubular target as a heat source positioned beneath the tubular target traverses along the latter's length. This causes reaction products of the vapor stream to be deposited on, and fuse to, the interior surface of the tubular target. The deposited material has the same index of refraction as the tubular target, but a different composition. This reference also suggests that one may achieve the same effect by an outside vapor-phase oxidation process or an outside vapor-phase axial deposition process, but does not explicitly teach how this can be done.
U.S. Pat. No. 4,923,497 to Leber et al. is directed to the continuous manufacture of a vertically drawn silica starter tube. This process uses silicon dioxide, in particle form, in a closed crucible. Next, the softened silicon dioxide is drawn into a tube, by means of a shaping tool, through an opening in the bottom of the crucible. In this process, the closed crucible, and also a space immediately below where the tube is formed, are provided with a high hydrogen-containing atmosphere. In addition, a predetermined electrical potential difference is maintained between the shaping tool and the crucible to create an electrical field therebetween to reduce impurities.
U.S. Pat. No. 5,026,413 to Leber et al. is also directed to the manufacture of a vertically drawn silica tube. High silica-containing quartz is softened in a furnace and drawn into a tube through an opening in the crucible's bottom. The pressure inside the tube, and the pressure inside a chamber into which the tube is formed, are monitored and equalized to minimize the diameter deviation of the tube.
U.S. Pat. No. 5,522,007 to Drouart et al. teaches the use of plasma deposition to build up an optical fiber preform having high hydroxyl ion concentration. In this reference, hydroxyl ions are deliberately entrained in a plasma generating gas by passing the gas through a water tank before it is introduced into one end of a plasma torch having an induction coil. The plasma torch projects molten silica particles mixed with hydroxyl ions onto a rotating substrate preform. This results in a preform having an average hydroxyl ion concentration lying in the range of 50-100 ppm deposited onto the target preform. According to Drouart et al., this technique results in optical fibers having an attenuation of 0.32 dB/km and 0.195 db/km at 1310 nm and 1550 nm, respectively.
U.S. Pat. No. 5,609,666 to Heitmann teaches the use of a tubular substrate formed from a porous oxide ceramic to form a quartz glass tube. A burner operated with a mixture of methane, silicon tetrachloride SiCl.sub.4 and oxygen is moved back and forth along the tubular substrate to deposit glass soot thereon. Simultaneously, a drying gas mixture comprising chlorine or thionyl chloride, along with other gases, is passed through the interior of the tubular substrate along the latter's axis. The purge gas removes the hydroxyl ions from the deposited glass soot. The deposited, purged glass soot body is removed from the tubular substrate and then subjected to further drying and sintering to form a low -OH concentration tube or rod.
The above processes all have disadvantages. First, tubes formed in a continuous process by drawing have high impurity levels, inclusions and, most often, a high hydroxyl content. Such tubes do not provide the desired mechanical and optical characteristics for the manufacture of optical fibers. Second, processes which call for soot deposition, followed by subsequent drying and sintering are expensive, and take a longer time, as they require two distinct steps which often cannot be carried out simultaneously.